Preheat the oven to 350 degrees F (175 degrees C). Bring a large pot of lightly salted water to a boil. Add pasta and cook until almost tender, about 10 minutes.
Fry the sausages in a large skillet over medium heat, turning frequently until cooked through, about 15 minutes. Remove from the skillet, cool slightly and slice into rounds. Add the garlic, fennel and onion to the skillet and season with salt and pepper. Cook and stir for about 5 minutes, then add the roasted red peppers, basil, sliced sausage and pasta sauce. Heat through over low heat until warmed.
Combine the pasta with the sauce and vegetables in a 9x13 inch baking dish. Spread the mozzarella, Parmesan and Asiago cheeses over the top. Garnish with a few fennel leaves left from the bulb. Cover with aluminum foil.
Bake for 30 minutes in the preheated oven, then remove the aluminum foil. Set the oven to broil, and cook for another 5 minutes or until cheese is browned.